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Effect of extrusion temperature on microstructure, thermal conductivity and mechanical properties of a Mg-Ce-Zn-Zr alloy
Journal of Alloys and Compounds 741 (2018) 1222e1228
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of extrusion temperature on microstructure, thermal conductivity and mechanical properties of a Mg-Ce-Zn-Zr alloy Ling-Fei Hu a, Qin-Fen Gu b, Qian Li a, c, Jie-Yu Zhang a, *, Guang-Xin Wu a, ** a State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China b Australian Synchrotron, 800 Blackburn Rd, Clayton, 3168, Australia c Materials Genome Institute, Shanghai University, Shanghai 200444, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2017 Received in revised form 7 January 2018 Accepted 14 January 2018
Mg-0.80Ce-0.69Zn-0.03Zr (wt %) alloys were prepared in the state of as-cast, by homogenization treatment at 713 K for 12 h and extruded at 573 K, 623 K, and 673 K with an extrusion ratio of 10.24: 1. Experimental results reveal that the grains are refined obviously after extrusion. With the increase of extrusion temperature, Zr-containing bar of the extruded alloy is broadened and the texture is weakened, which reduces the anisotropy of thermal conductivity. These phenomena are attributed to the work hardening, recovery, recrystallization, and grain growth, which depend deeply on extrusion temperature. The obvious increase of Ce in Mg matrix according to thermodynamic calculation may act as a major role in the thermal conductivity decreasing of extruded Mg-Ce-Zn-Zr alloy for extrusion temperature increasing from 623 K to 673 K. The recrystallization and grain growth can be driven more effectively at a higher temperature, which should be responsible for the decrease of tensile strength and the improvement of elongation. © 2018 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Extrusion Texture Thermal conductivity Mechanical properties
1. Introduction Magnesium alloys are widely used in aerospace, automobile, and 3C products due to their low density, high specific strength, and good thermal property. Furthermore, the structural magnesium with good heat dissipation ability is widely required in LED lights, cryogenic, and integrated circuits for saving weight [1e4]. Many ZK series commercial magnesium alloys have been widely used because of their excellent performance in strength and ductility, which are attributed to the solution ability of Zn and grain refinement of Zr in Mg [5]. Zn and Zr can act in a number of ways to improve corrosion resistance [6,7]. et al. In the aspect of thermal properties, although Rudajevova [8,9] reported that solid solubility had a great influence on thermal conductivity of alloy, Zheng et al. [10] found that Zn addition in Mg alloy hardly changed the number of electrons per atom, or the
* Corresponding author. at: State Key Laboratory of Advanced Special Steels, Shanghai University, Shanghai 200444, China. ** Corresponding author. State Key Laboratory of Advanced Special Steels, Shanghai University, Shanghai 200444, China. E-mail addresses: [email protected] (J.-Y. Zhang), [email protected] (G.-X. Wu). https://doi.org/10.1016/j.jallcom.2018.01.203 0925-8388/© 2018 Elsevier B.V. All rights reserved.
shape of the Brillouin zone was unchanged, since Zn is the weakest electron scatterer. Zhang et al. [11] reported that thermal conductivity of T4 MgeZneMn alloys were higher than those of as-cast alloys, which may be attributed to the dissolution of MgeZn phase precipitated from the Mg matrix after solution treatment. Thus, a limited addition of Zn in Mg alloy will bring solution strengthening effect, rather than make a big difference in thermal conductivity of alloy. On the other hand, with the addition of Ce in magnesium alloy, heat resistance and thermal property can be improved, which is mainly attributed to the low solid solution of Ce in magnesium and the precipitation of Ce containing intermetallic [5,12,13]. Moreover, since earing is a well-known industrial problem caused by crystallographic texture and anisotropy of plastic deformation [14], Ce is always alloyed to magnesium to weaken the texture, which is attributed to the dynamic recrystallization (DRX) caused by particle stimulated nucleation (PSN) at Mg-Ce particles [15e18]. It is widely reported that extrudability and mechanical properties of the Mg extrusions can be significantly improved with a 0.2 wt % Ce addition, due to grain refinement and dispersion strengthening provided by the Mg12Ce particles, homogeneous deformation due to non-basal slips and the beneficial texture [19e21]. Ce is also the lightest rare earth element with a low price, which may contribute to the industrialization of magnesium alloys.
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It seems that Ce, Zn, and Zr are potential elements to develop a magnesium alloy with high thermal conductivity and mechanical properties. However, it has not paid enough attention to the mechanism of rare earth element Ce addition on Mg-Zn-Zr heat dissipation magnesium alloy. An indirect extrusion as an effective way to process magnesium has many advantages, for instance, it is free of lubricant in the wall of container [22]. Moreover, the microstructure evolution related to process parameters is important to material design besides the optimization of chemical composition [23,24]. The anisotropy of thermal conductivity and mechanical properties in the extruded magnesium alloy caused by texture was investigated in Refs. [25] and [26]. Meanwhile, the extrusion temperature has a significant effect on microstructure evolution, thermal and mechanical properties. Peng et al. [1] reported that the thermal conductivity of Mg2.0Zn-1.0Mn-0.2Ce increased initially and then decreased with the increase of extrusion temperature, due to the grain size and solid solution of Mn. It was claimed that grain boundary and solution of Mn in Mg can be scattering centers of electrons and phonons. However, few researches on the comprehensive influence of extrusion temperature on microstructure evolution, thermal conductivity, and mechanical properties of Ce, Zn, and Zr containing magnesium alloys have been carried out. This study aims to systematically investigate the effect of extrusion temperature on microstructure evolution and the relationship among microstructure, mechanical properties, and thermal conductivity. 2. Experiment The as-cast billet was provided by Baotou Research Institute of Rare Earths. The actual chemical compositions of the samples were measured by Inductively Coupled Plasma atomic emission spectrometry (ICP-AES), and the results are listed in Table 1. It showed that little Zr was dissolved in the alloy during casting. The casted ingot was processed into several billets with a diameter of 80 mm and a length of 44 mm using electrical-discharge machining (EDM). All the billets were homogenized at 713 K for 12 h in a resistance furnace and then quenched in water. In order to stabilize at extrusion temperature, the billets and die were preheated for 40 min at 573 K, 623 K, and 673 K, respectively. Indirect extrusion was conducted on a lab-scale 315 ton four-column hydraulic machine with a ram speed of 1 mm/s, and an extrusion rate of 10.24. After extrusion, the sample was quenched in ice water soon. The rod-like extrusion profiles were 25 mm in diameter and nearly 30 cm in length as shown in Fig. 1. Microstructure morphology was observed on Apollo 300 equipped with energy dispersive spectrometer (EDS) and electron backscattered diffraction (EBSD). Samples for EBSD examination were ground on 4000 grit silicon carbide paper and polished using 1 mm diamond polishing solution, followed by a final polish with colloidal silica for 30 min. Channel 5 (HKL Technology, Denmark) was used to analyze the data of EBSD. The samples parallel to the extrusion direction (ED) were used for EBSD and SEM analysis. Thermal conductivity was measured using NETZSCH-LFA447 flash thermal conductivity analyzer based on laser flash method described in Ref. [27]. Discs with a thickness of 1 mm and diameter
Table 1 Chemical composition of the studied Mg-Ce-Zn-Zr alloy. Nominal alloy
Composition (wt %) Mg
Ce
Zn
Zr
Mg-1.0Ce-0.5Zn-0.5Zr
Bal.
0.80
0.69
0.03
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Fig. 1. Samples extruded at 573 K, 623 K and 673 K.
of 12.7 mm cut from the extruded rod were used for thermal diffusivity properties test. The thermal conductivity was calculated from the formula below [5].
l ¼ Cp ar
(1)
Where l is thermal conductivity, Cp is the specific heat capacity, a is the thermal diffusivity, r is the density. To learn the relation between extrusion temperature and thermal conductivity anisotropy, three different orientation specimens with tilt angles of 0 , 45 , 90 between extrusion direction of each extrusion temperature were tested. Tensile properties were tested by Instron with an initial strain rate of 1 mm/min. The plate samples with a gauge length of 25 mm and gauge width of 6.25 cm were used for testing. The high-resolution XRD (HRXRD) patterns of samples of as-cast and homogenized at 713 K for 12 h were collected by a Mythen-II detector on the powder diffraction beamline at Australian synchrotron with l ¼ 0.09995 nm, refinement with Topas 4.2 based on Rietveld. 3. Results and discussion 3.1. Microstructure of as-cast and as-homogenized alloys The morphology of the as-cast alloy revealed that there existed two phases, the matrix a-Mg and a network like eutectic products around the boundary (Fig. 2a). The eutectic products are determined to be Mg-Ce-Zn phases through EDS analysis (Fig. 3aef). After homogenization at 713 K for 12 h, the network-like intermetallic becomes interrupted, and cluster particles precipitated in the center of the grains (Fig. 2b). Compared with the phase around the boundary, Zn content of interrupted network-like intermetallic was decreased in the as-homogenized samples according to EDS analysis (Fig. 3gel). Particles in the middle of grain are Zr containing phase, which is consistent with the results investigated by Peng and Chang et al. [28,29]. For a limit content of Mg-Ce-Zn phase in the as-cast alloy and Mg-Ce, Zr-rich phase in the as-homogenized alloy, the higher resolution will be achieved from the analyzed data of synchrotron source XRD. The HRXRD results show that the matrix a-Mg and intermetallic phase (Mg, Zn)12Ce existed in the as-cast alloy, and (Mg, Zn)12Ce transformed to be (Mg0.75Ce0.25)O2 after homogenized treatment. The existence of oxygen in (Mg0.75Ce0.25)O2 may relate to the oxidation of sample after homogenization. Zr-rich phase is not detected, possibly due to its low content. The crystal structure of (Mg, Zn)12Ce and (Mg0.75Ce0.25)O2 is analyzed, and the crystal
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Fig. 2. (a) As-cast and (b) As-homogenized microstructure.
Fig. 3. Magnified SEM images of as-casted (a) and as-homogenized (b) Mg-Ce-Zn-Zr alloy and corresponding EDS mapping images of (e) Mg, (d) Ce, (e) Zn, (f) Zr in (a) and (i) Mg, (j) Ce, (k) Zn, (l) Zr in (a). (b) is the point results of point 1, 2 in (a) and (h) is the point results of point 1, 2 in (b).
L.-F. Hu et al. / Journal of Alloys and Compounds 741 (2018) 1222e1228
structure models are built (Fig. 4a and b), which indicates that the Mg-Ce-Zn phase in the as-cast alloy is in a Mg12Ce-based structure with parts of Zn substitution for Mg. The zinc diffuses into the matrix during homogenized treatment, which corresponds to the report of Kevorkov et al. [30]. 3.2. Microstructure and texture of as-extruded alloys The SEM observation results of samples extruded at different temperature are shown in Fig. 5. The Mg-Ce intermetallic and Zr rich phase are elongated. When Mg-Ce-Zn-Zr extruded at a low temperature, the Mg-Ce intermetallic breaks and the Zr-rich bar is slim and concentrated. Otherwise, when Mg-Ce-Zn-Zr extruded at a higher temperature, the Mg-Ce intermetallic stays completed and Zr rich phase becomes wide and dispersed. To understand the correlation between extrusion temperature and texture evolution, the EBSD results are shown in Fig. 6. Compared with the microstructure of as-cast and as-homogenized samples, except for the Zr rich bar area extruded at 573 K and 623 K, the grains in the most parts of the as-extruded alloy are heavily refined attribute to recrystallization. The texture of crystallographic plane {1210} is weakened with the increasing extrusion temperature. However, the recrystallization does not occur around the area of Zr rich bar at low extrusion temperature. It may be due to the pinning of fine Zr-rich particles (less than 1 mm as shown in Fig. 2b) in the sub-grain that prevent grain growth, resulting in the inhibition of recrystallization refer to Zener pinning mechanism [31]. Meanwhile, higher driving force can be obtained with the increasing of extrusion temperature [32,33], which will induce the
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recrystallization in the area of fine Zr rich phase, as shown in Fig. 6. 3.3. The thermal conductivity and anisotropy of the as-extruded alloys The thermal conductivity testing results according to LFA methods can be seen in Fig. 7. The anisotropy of thermal conductivity of samples extruded at three different temperature can be described by a standard deviation of sample estimate (STEDV) line based on thermal conductivity results of samples with tilt angles of 0 , 45 , and 90 between extrusion direction. The tendency of the thermal conductivity anisotropy is consistent with the trend of texture variation. When the alloys are extruded at a higher temperature, the texture of the Mg-Ce-Zn-Zr alloy is weakened, and the thermal conductivity tends to be isotropic. The thermal conductivity of the alloy initially increased, as the extrusion temperature rising from 573 K to 623 K, and it reaches 180.2 W/(m*K) on average. However, when the extrusion temperature increased to 673 K, the thermal conductivity of the alloy is decreased to 134.7 W/(m*K) on average. Recrystallization is initially inhibited in Zr-rich bar at low extrusion temperature and occurred at higher extrusion temperature. It indicates that work hardening, recovery, and recrystallization simultaneously affected the microstructure evolution when the alloy extruded at an elevated temperature. The extent of each factor is mainly determined by extrusion temperature. Work hardening dominated at low extrusion temperature, and recovery, as well as recrystallization, gradually took the place of work hardening with the increasing extrusion temperature, as reported by Kun [34]. Thus, the alloy
Fig. 4. HRXRD analysis results (a) As-cast (b) As-homogenized.
Fig. 5. Alloy microstructure after extrusion: (a) Extrusion at 573 K (b) Extrusion at 623 K (c) Extrusion at 673 K
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Fig. 6. EBSD maps (aec) and inverse pole figures (dee) from longitudinal section of extruded sample for different extrusion temperature: (a, d) 573 K (b, e) 623 K (c, f) 673 K.
decreases as the increase of Ce addition [35], although the solubility of Ce in the Mg alloy is a trace amount. Because of the solubility of Zr in Mg matrix is quite small, Mg-Ce-Zn phase diagrams in Mg rich corner combined with point calculation (point calculation can be used to calculate the value of solid solubility under different temperature) at 573 K, 623 K, and 673 K calculated by Pandat software were used to learn the solubility difference of Zn and Ce in Mg. It can be observed that the solid solubility of Ce in Mg has an obvious increase with the temperature increasing from 623 K to 673 K (Fig. 8). The fraction of Ce in a-Mg increased from 0.00383 wt % to 0.01302 wt % as shown in Table 2. As the temperature increases from 573 K to 673 K, the increasing extent of Zn solution in Mg is small because of the low content of Zn in as-received alloy compared with high solid solubility of Zn in Mg. Thus dissolved Ce in Mg matrix plays a major role in decreasing the thermal conductivity as the extrusion temperature increases from 623 K to
Fig. 7. Thermal conductivity results of different orientation in extrusion samples.
extruded at low temperature contained more dislocations than that extruded at a higher temperature. Dislocation will act as scattering centers for electrons and phonons, which cause the decrease of thermal conductivity [1,26,35]. Thus, the thermal conductivity of as-extruded Mg-Ce-Zn-Zr increased from 573 K to 623 K, which is attributed to the weakening of work hardening, in contrast with the alloys extruded at a higher temperature of 623 K. However, when the extrusion temperature reaches to 673 K, the thermal conductivity of the as-extruded alloys is lower than the alloys extruded at 623 K. It may be attributed to the disappearance of work hardening when the extrusion temperature exceeds 623 K, as well as the obvious increase of Ce content dissolved in Mg. It is widely accepted that alloy elements dissolved in the matrix will increase the electron scattering more obviously, thereby reducing the thermal conductivity. Moreover, it is also confirmed by Peng that the thermal conductivity of the magnesium alloy still
Fig. 8. Mg-Ce-Zn alloy phase diagram in Mg -rich corner of 573 K (the green line), 623 K (the blue line) and 673 K (the red line) calculated by Pandat software. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
L.-F. Hu et al. / Journal of Alloys and Compounds 741 (2018) 1222e1228 Table 2 The point calculation results of solid solubility of Ce and Zn in Mg of the studied MgCe-Zn-Zr alloy calculated by Pandat software. wt %
573 K
623 K
673 K
Fraction of Ce in a-Mg (wt %) Fraction of Zn in a-Mg (wt %)
0.00090 0.29641
0.00383 0.33362
0.01302 0.36782
673 K. Moreover, more complete recrystallization played at the extrusion temperature higher than 623 K, which indicates more equiaxed grain would form as well as more grain boundaries. The grain boundary can also be the factor to block the electron and phonons transfer [1] ).
3.4. The mechanical properties of as-extruded alloys The mechanical properties of as-cast, as-homogenized and asextruded are shown in Fig. 9. Compared with the as-casted alloy, the as-extruded one have a higher elongation because of the segregation reduction. The ultimate tensile strain (UTS) and elongation are increased obviously after extrusion. Finer grains play an important role in improving the mechanical properties of asextruded alloys according to Hall-Petch formula. With the increasing extrusion temperature, the UTS of the Mg-Ce-Zn-Zr alloys rose first and then decreased while the elongation was inverse. Complex mechanism based on work hardening, recovery and recrystallization during the process of extrusion should be responsible for the change of UTS refer to [34]. Work hardening works at low extrusion temperature as mentioned before while recovery and recrystallization act at higher extrusion temperature. Work hardening works in Mg-Ce-Zn-Zr alloys when the extrusion temperature was lower than 623 K. Dislocation combined with refined grains lead to higher strength and lower elongation. When the dislocation caused by work hardening disappeared and the grain grows heavily in the extrusion temperature higher than 623 K, the strength decreased whereas the elongation increased.
4. Conclusion The influence of microstructure evolution on mechanical properties and thermal conductivity of Mg-Ce-Zn-Zr alloy extruded at different temperatures is investigated. Based on the analysis results, the conclusions are obtained as follows:
Fig. 9. Tensile testing results of as-casted, as-homogenized and as-extruded samples.
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1. Two phases Mg matrix and (Mg, Zn)12Ce existed in the as-cast alloy. Zr-core appeared and grain boundary interrupted with the homogenization treatment at 713 K for 12 h, and the (Mg, Zn)12Ce transforms to (Mg0.75Ce0.25)O2. 2. With the increase of extrusion temperature, the Zr-containing bar is broadened and the texture of alloy is weakened due to recovery, recrystallization and grain growth, the thermal conductivity of extruded alloys changed from isotropic to anisotropy. The thermal conductivity of Mg-Ce-Zn-Zr initially increased and then decreased when the extrusion temperature increased from 573 K to 673 K because of the disappearance of dislocation and the increased solid solution of Ce. 3. Compared with the as-cast and as-homogenized state, the grain refined with the process of extrusion which contributes to a higher UTS and elongation. The UTS in the longitudinal direction of extruded alloys rose first and then decreased with the increase of extrusion temperature, while the change of elongation is inverse, due to the work hardening, grain refinement, and growth. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51474143, 51674163 and 51074103) and Science and Technology Committee of Shanghai under No. 16ZR1412000. The authors gratefully acknowledge support for experiment materials from Baotou Research Institute of Rare Earths, material analysis and research from Instrumental Analysis and Research Center of Shanghai University, SR-PXRD analysis from Powder diffraction beamline, Australian Synchrotron. References [1] J. Peng, L. Zhong, Y. Wang, et al., Effect of extrusion temperature on the microstructure and thermal conductivity of Mge2.0Zne1.0Mne0.2Ce alloys, Mater. Des. 87 (6) (2015) 914e919. [2] C. Wang, Z. Cui, H. Liu, et al., Electrical and thermal conductivity in Mge5Sn alloy at different aging status, Mater. Des. 84 (2015) 48e52. [3] I. Ostrovsky, Y. Henn, Present State and Future of Magnesium Application in Aerospace Industry, 2007. [4] M. Galeazzi, D.F. Bogorin, K. Prasai, et al., Note: thermal properties of magnesium in the 60-150 mK range, Rev. Sci. Instrum. 81 (7) (2010), 076105. [5] H. Yu, Y.M. Kim, B.S. You, et al., Effects of cerium addition on the microstructure, mechanical properties and hot workability of ZK60 alloy, Mater. Sci. Eng. 559 (598) (2013) 798e807. [6] G. Song, D. StJohn, The effect of zirconium grain refinement on the corrosion behaviour of magnesium-rare earth alloy MEZ, J. Light Met. 2 (1) (2002) 1e16. [7] H. Haferkamp, F. Bach, V. Kaese, et al., Magnesium corrosioneprocesses, protection of anode and cathode, Magn. Alloys Technol. (2004) 226e241. , M. Stane k, P. Luka [8] A. Rudajevova c, Determination of thermal diffusivity and thermal conductivity of Mg-Al alloys, Mater. Sci. Eng. A 341 (1e2) (2003) 152e157. [9] Y.M. Kim, S.W. Choi, S.K. Hong, The behavior of thermal diffusivity change according to the heat treatment in Al-Si binary system, J. Alloys Compd. 687 (2016) 54e58. [10] T. Ying, H. Chi, M. Zheng, et al., Low-temperature electrical resistivity and thermal conductivity of binary magnesium alloys, Acta Mater. 80 (2014) 288e295. [11] J. Yuan, K. Zhang, X. Zhang, et al., Thermal characteristics of MgeZneMn alloys with high specific strength and high thermal conductivity, J. Alloys Compd. 578 (6) (2013) 32e36. [12] L. Zhong, J. Peng, M. Li, et al., Effect of Ce addition on the microstructure, thermal conductivity and mechanical properties of Mge0.5Mn alloys, J. Alloys Compd. 661 (2016) 402e410. [13] J. Peng, L. Zhong, Y. Wang, et al., Effect of Ce addition on thermal conductivity of Mge2Zne1Mn alloy, J. Alloys Compd. 639 (5) (2015) 556e562. [14] T. Walde, H. Riedel, Simulation of earing during deep drawing of magnesium alloy AZ31, Acta Mater. 55 (3) (2007) 867e874. [15] R.K. Mishra, A.K. Gupta, P.R. Rao, et al., Influence of cerium on the texture and ductility of magnesium extrusions, Scripta Mater. 59 (5) (2008) 562e565. [16] R.K. Sabat, R.K. Mishra, A.K. Sachdev, et al., The deciding role of texture on ductility in a Ce containing Mg alloy, Mater. Lett. 153 (2015) 158e161. [17] N. Stanford, D. Atwell, A. Beer, et al., Effect of microalloying with rare-earth elements on the texture of extruded magnesium-based alloys, Scripta Mater. 59 (7) (2008) 772e775.
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[18] J. Bohlen, M.R. Nürnberg, J.W. Senn, et al., The texture and anisotropy of magnesiumezincerare earth alloy sheets, Acta Mater. 55 (6) (2007) 2101e2112. [19] A.A. Luo, W. Wu, R.K. Mishra, et al., Microstructure and mechanical properties of extruded magnesium-aluminum-cerium alloy tubes, Metall. Mater. Trans. A 41 (10) (2010) 2662e2674. [20] Y. Chino, M. Kado, M. Mabuchi, Compressive deformation behavior at room temperature e 773 K in Mge0.2 mass%(0.035at.%)Ce alloy, Acta Mater. 56 (3) (2008) 387e394. [21] R.K. Mishra, A.K. Gupta, P.R. Rao, et al., Influence of cerium on texture and ductility of magnesium extrusions, Scripta Mater. 59 (5) (2008) 562e565. [22] S.H. Park, H. Yu, J.H. Bae, et al., Microstructural evolution of indirect-extruded ZK60 alloy by adding Ce, J. Alloys Compd. 545 (51) (2012) 139e143. [23] M. Shahzad, L. Wagner, Influence of extrusion parameters on microstructure and texture developments, and their effects on mechanical properties of the magnesium alloy AZ80, Mater. Sci. Eng. A 506 (1e2) (2009) 141e147. [24] M. Yamasaki, Y. Kawamura, Thermal diffusivity and thermal conductivity of MgeZnerare earth element alloys with long-period stacking ordered phase, Scripta Mater. 60 (4) (2009) 264e267. [25] J. Yuan, K. Zhang, T. Li, et al., Anisotropy of thermal conductivity and mechanical properties in Mge5Zne1Mn alloy, Mater. Des. 40 (2012) 257e261. [26] T. Ying, M.Y. Zheng, Z.T. Li, et al., Thermal conductivity of as-cast and asextruded binary MgeZn alloys, J. Alloys Compd. 621 (38) (2015) 250e255.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of extrusion temperature on microstructure, thermal conductivity and mechanical properties of a Mg-Ce-Zn-Zr alloy Ling-Fei Hu a, Qin-Fen Gu b, Qian Li a, c, Jie-Yu Zhang a, *, Guang-Xin Wu a, ** a State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China b Australian Synchrotron, 800 Blackburn Rd, Clayton, 3168, Australia c Materials Genome Institute, Shanghai University, Shanghai 200444, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2017 Received in revised form 7 January 2018 Accepted 14 January 2018
Mg-0.80Ce-0.69Zn-0.03Zr (wt %) alloys were prepared in the state of as-cast, by homogenization treatment at 713 K for 12 h and extruded at 573 K, 623 K, and 673 K with an extrusion ratio of 10.24: 1. Experimental results reveal that the grains are refined obviously after extrusion. With the increase of extrusion temperature, Zr-containing bar of the extruded alloy is broadened and the texture is weakened, which reduces the anisotropy of thermal conductivity. These phenomena are attributed to the work hardening, recovery, recrystallization, and grain growth, which depend deeply on extrusion temperature. The obvious increase of Ce in Mg matrix according to thermodynamic calculation may act as a major role in the thermal conductivity decreasing of extruded Mg-Ce-Zn-Zr alloy for extrusion temperature increasing from 623 K to 673 K. The recrystallization and grain growth can be driven more effectively at a higher temperature, which should be responsible for the decrease of tensile strength and the improvement of elongation. © 2018 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Extrusion Texture Thermal conductivity Mechanical properties
1. Introduction Magnesium alloys are widely used in aerospace, automobile, and 3C products due to their low density, high specific strength, and good thermal property. Furthermore, the structural magnesium with good heat dissipation ability is widely required in LED lights, cryogenic, and integrated circuits for saving weight [1e4]. Many ZK series commercial magnesium alloys have been widely used because of their excellent performance in strength and ductility, which are attributed to the solution ability of Zn and grain refinement of Zr in Mg [5]. Zn and Zr can act in a number of ways to improve corrosion resistance [6,7]. et al. In the aspect of thermal properties, although Rudajevova [8,9] reported that solid solubility had a great influence on thermal conductivity of alloy, Zheng et al. [10] found that Zn addition in Mg alloy hardly changed the number of electrons per atom, or the
* Corresponding author. at: State Key Laboratory of Advanced Special Steels, Shanghai University, Shanghai 200444, China. ** Corresponding author. State Key Laboratory of Advanced Special Steels, Shanghai University, Shanghai 200444, China. E-mail addresses: [email protected] (J.-Y. Zhang), [email protected] (G.-X. Wu). https://doi.org/10.1016/j.jallcom.2018.01.203 0925-8388/© 2018 Elsevier B.V. All rights reserved.
shape of the Brillouin zone was unchanged, since Zn is the weakest electron scatterer. Zhang et al. [11] reported that thermal conductivity of T4 MgeZneMn alloys were higher than those of as-cast alloys, which may be attributed to the dissolution of MgeZn phase precipitated from the Mg matrix after solution treatment. Thus, a limited addition of Zn in Mg alloy will bring solution strengthening effect, rather than make a big difference in thermal conductivity of alloy. On the other hand, with the addition of Ce in magnesium alloy, heat resistance and thermal property can be improved, which is mainly attributed to the low solid solution of Ce in magnesium and the precipitation of Ce containing intermetallic [5,12,13]. Moreover, since earing is a well-known industrial problem caused by crystallographic texture and anisotropy of plastic deformation [14], Ce is always alloyed to magnesium to weaken the texture, which is attributed to the dynamic recrystallization (DRX) caused by particle stimulated nucleation (PSN) at Mg-Ce particles [15e18]. It is widely reported that extrudability and mechanical properties of the Mg extrusions can be significantly improved with a 0.2 wt % Ce addition, due to grain refinement and dispersion strengthening provided by the Mg12Ce particles, homogeneous deformation due to non-basal slips and the beneficial texture [19e21]. Ce is also the lightest rare earth element with a low price, which may contribute to the industrialization of magnesium alloys.
L.-F. Hu et al. / Journal of Alloys and Compounds 741 (2018) 1222e1228
It seems that Ce, Zn, and Zr are potential elements to develop a magnesium alloy with high thermal conductivity and mechanical properties. However, it has not paid enough attention to the mechanism of rare earth element Ce addition on Mg-Zn-Zr heat dissipation magnesium alloy. An indirect extrusion as an effective way to process magnesium has many advantages, for instance, it is free of lubricant in the wall of container [22]. Moreover, the microstructure evolution related to process parameters is important to material design besides the optimization of chemical composition [23,24]. The anisotropy of thermal conductivity and mechanical properties in the extruded magnesium alloy caused by texture was investigated in Refs. [25] and [26]. Meanwhile, the extrusion temperature has a significant effect on microstructure evolution, thermal and mechanical properties. Peng et al. [1] reported that the thermal conductivity of Mg2.0Zn-1.0Mn-0.2Ce increased initially and then decreased with the increase of extrusion temperature, due to the grain size and solid solution of Mn. It was claimed that grain boundary and solution of Mn in Mg can be scattering centers of electrons and phonons. However, few researches on the comprehensive influence of extrusion temperature on microstructure evolution, thermal conductivity, and mechanical properties of Ce, Zn, and Zr containing magnesium alloys have been carried out. This study aims to systematically investigate the effect of extrusion temperature on microstructure evolution and the relationship among microstructure, mechanical properties, and thermal conductivity. 2. Experiment The as-cast billet was provided by Baotou Research Institute of Rare Earths. The actual chemical compositions of the samples were measured by Inductively Coupled Plasma atomic emission spectrometry (ICP-AES), and the results are listed in Table 1. It showed that little Zr was dissolved in the alloy during casting. The casted ingot was processed into several billets with a diameter of 80 mm and a length of 44 mm using electrical-discharge machining (EDM). All the billets were homogenized at 713 K for 12 h in a resistance furnace and then quenched in water. In order to stabilize at extrusion temperature, the billets and die were preheated for 40 min at 573 K, 623 K, and 673 K, respectively. Indirect extrusion was conducted on a lab-scale 315 ton four-column hydraulic machine with a ram speed of 1 mm/s, and an extrusion rate of 10.24. After extrusion, the sample was quenched in ice water soon. The rod-like extrusion profiles were 25 mm in diameter and nearly 30 cm in length as shown in Fig. 1. Microstructure morphology was observed on Apollo 300 equipped with energy dispersive spectrometer (EDS) and electron backscattered diffraction (EBSD). Samples for EBSD examination were ground on 4000 grit silicon carbide paper and polished using 1 mm diamond polishing solution, followed by a final polish with colloidal silica for 30 min. Channel 5 (HKL Technology, Denmark) was used to analyze the data of EBSD. The samples parallel to the extrusion direction (ED) were used for EBSD and SEM analysis. Thermal conductivity was measured using NETZSCH-LFA447 flash thermal conductivity analyzer based on laser flash method described in Ref. [27]. Discs with a thickness of 1 mm and diameter
Table 1 Chemical composition of the studied Mg-Ce-Zn-Zr alloy. Nominal alloy
Composition (wt %) Mg
Ce
Zn
Zr
Mg-1.0Ce-0.5Zn-0.5Zr
Bal.
0.80
0.69
0.03
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Fig. 1. Samples extruded at 573 K, 623 K and 673 K.
of 12.7 mm cut from the extruded rod were used for thermal diffusivity properties test. The thermal conductivity was calculated from the formula below [5].
l ¼ Cp ar
(1)
Where l is thermal conductivity, Cp is the specific heat capacity, a is the thermal diffusivity, r is the density. To learn the relation between extrusion temperature and thermal conductivity anisotropy, three different orientation specimens with tilt angles of 0 , 45 , 90 between extrusion direction of each extrusion temperature were tested. Tensile properties were tested by Instron with an initial strain rate of 1 mm/min. The plate samples with a gauge length of 25 mm and gauge width of 6.25 cm were used for testing. The high-resolution XRD (HRXRD) patterns of samples of as-cast and homogenized at 713 K for 12 h were collected by a Mythen-II detector on the powder diffraction beamline at Australian synchrotron with l ¼ 0.09995 nm, refinement with Topas 4.2 based on Rietveld. 3. Results and discussion 3.1. Microstructure of as-cast and as-homogenized alloys The morphology of the as-cast alloy revealed that there existed two phases, the matrix a-Mg and a network like eutectic products around the boundary (Fig. 2a). The eutectic products are determined to be Mg-Ce-Zn phases through EDS analysis (Fig. 3aef). After homogenization at 713 K for 12 h, the network-like intermetallic becomes interrupted, and cluster particles precipitated in the center of the grains (Fig. 2b). Compared with the phase around the boundary, Zn content of interrupted network-like intermetallic was decreased in the as-homogenized samples according to EDS analysis (Fig. 3gel). Particles in the middle of grain are Zr containing phase, which is consistent with the results investigated by Peng and Chang et al. [28,29]. For a limit content of Mg-Ce-Zn phase in the as-cast alloy and Mg-Ce, Zr-rich phase in the as-homogenized alloy, the higher resolution will be achieved from the analyzed data of synchrotron source XRD. The HRXRD results show that the matrix a-Mg and intermetallic phase (Mg, Zn)12Ce existed in the as-cast alloy, and (Mg, Zn)12Ce transformed to be (Mg0.75Ce0.25)O2 after homogenized treatment. The existence of oxygen in (Mg0.75Ce0.25)O2 may relate to the oxidation of sample after homogenization. Zr-rich phase is not detected, possibly due to its low content. The crystal structure of (Mg, Zn)12Ce and (Mg0.75Ce0.25)O2 is analyzed, and the crystal
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Fig. 2. (a) As-cast and (b) As-homogenized microstructure.
Fig. 3. Magnified SEM images of as-casted (a) and as-homogenized (b) Mg-Ce-Zn-Zr alloy and corresponding EDS mapping images of (e) Mg, (d) Ce, (e) Zn, (f) Zr in (a) and (i) Mg, (j) Ce, (k) Zn, (l) Zr in (a). (b) is the point results of point 1, 2 in (a) and (h) is the point results of point 1, 2 in (b).
L.-F. Hu et al. / Journal of Alloys and Compounds 741 (2018) 1222e1228
structure models are built (Fig. 4a and b), which indicates that the Mg-Ce-Zn phase in the as-cast alloy is in a Mg12Ce-based structure with parts of Zn substitution for Mg. The zinc diffuses into the matrix during homogenized treatment, which corresponds to the report of Kevorkov et al. [30]. 3.2. Microstructure and texture of as-extruded alloys The SEM observation results of samples extruded at different temperature are shown in Fig. 5. The Mg-Ce intermetallic and Zr rich phase are elongated. When Mg-Ce-Zn-Zr extruded at a low temperature, the Mg-Ce intermetallic breaks and the Zr-rich bar is slim and concentrated. Otherwise, when Mg-Ce-Zn-Zr extruded at a higher temperature, the Mg-Ce intermetallic stays completed and Zr rich phase becomes wide and dispersed. To understand the correlation between extrusion temperature and texture evolution, the EBSD results are shown in Fig. 6. Compared with the microstructure of as-cast and as-homogenized samples, except for the Zr rich bar area extruded at 573 K and 623 K, the grains in the most parts of the as-extruded alloy are heavily refined attribute to recrystallization. The texture of crystallographic plane {1210} is weakened with the increasing extrusion temperature. However, the recrystallization does not occur around the area of Zr rich bar at low extrusion temperature. It may be due to the pinning of fine Zr-rich particles (less than 1 mm as shown in Fig. 2b) in the sub-grain that prevent grain growth, resulting in the inhibition of recrystallization refer to Zener pinning mechanism [31]. Meanwhile, higher driving force can be obtained with the increasing of extrusion temperature [32,33], which will induce the
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recrystallization in the area of fine Zr rich phase, as shown in Fig. 6. 3.3. The thermal conductivity and anisotropy of the as-extruded alloys The thermal conductivity testing results according to LFA methods can be seen in Fig. 7. The anisotropy of thermal conductivity of samples extruded at three different temperature can be described by a standard deviation of sample estimate (STEDV) line based on thermal conductivity results of samples with tilt angles of 0 , 45 , and 90 between extrusion direction. The tendency of the thermal conductivity anisotropy is consistent with the trend of texture variation. When the alloys are extruded at a higher temperature, the texture of the Mg-Ce-Zn-Zr alloy is weakened, and the thermal conductivity tends to be isotropic. The thermal conductivity of the alloy initially increased, as the extrusion temperature rising from 573 K to 623 K, and it reaches 180.2 W/(m*K) on average. However, when the extrusion temperature increased to 673 K, the thermal conductivity of the alloy is decreased to 134.7 W/(m*K) on average. Recrystallization is initially inhibited in Zr-rich bar at low extrusion temperature and occurred at higher extrusion temperature. It indicates that work hardening, recovery, and recrystallization simultaneously affected the microstructure evolution when the alloy extruded at an elevated temperature. The extent of each factor is mainly determined by extrusion temperature. Work hardening dominated at low extrusion temperature, and recovery, as well as recrystallization, gradually took the place of work hardening with the increasing extrusion temperature, as reported by Kun [34]. Thus, the alloy
Fig. 4. HRXRD analysis results (a) As-cast (b) As-homogenized.
Fig. 5. Alloy microstructure after extrusion: (a) Extrusion at 573 K (b) Extrusion at 623 K (c) Extrusion at 673 K
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Fig. 6. EBSD maps (aec) and inverse pole figures (dee) from longitudinal section of extruded sample for different extrusion temperature: (a, d) 573 K (b, e) 623 K (c, f) 673 K.
decreases as the increase of Ce addition [35], although the solubility of Ce in the Mg alloy is a trace amount. Because of the solubility of Zr in Mg matrix is quite small, Mg-Ce-Zn phase diagrams in Mg rich corner combined with point calculation (point calculation can be used to calculate the value of solid solubility under different temperature) at 573 K, 623 K, and 673 K calculated by Pandat software were used to learn the solubility difference of Zn and Ce in Mg. It can be observed that the solid solubility of Ce in Mg has an obvious increase with the temperature increasing from 623 K to 673 K (Fig. 8). The fraction of Ce in a-Mg increased from 0.00383 wt % to 0.01302 wt % as shown in Table 2. As the temperature increases from 573 K to 673 K, the increasing extent of Zn solution in Mg is small because of the low content of Zn in as-received alloy compared with high solid solubility of Zn in Mg. Thus dissolved Ce in Mg matrix plays a major role in decreasing the thermal conductivity as the extrusion temperature increases from 623 K to
Fig. 7. Thermal conductivity results of different orientation in extrusion samples.
extruded at low temperature contained more dislocations than that extruded at a higher temperature. Dislocation will act as scattering centers for electrons and phonons, which cause the decrease of thermal conductivity [1,26,35]. Thus, the thermal conductivity of as-extruded Mg-Ce-Zn-Zr increased from 573 K to 623 K, which is attributed to the weakening of work hardening, in contrast with the alloys extruded at a higher temperature of 623 K. However, when the extrusion temperature reaches to 673 K, the thermal conductivity of the as-extruded alloys is lower than the alloys extruded at 623 K. It may be attributed to the disappearance of work hardening when the extrusion temperature exceeds 623 K, as well as the obvious increase of Ce content dissolved in Mg. It is widely accepted that alloy elements dissolved in the matrix will increase the electron scattering more obviously, thereby reducing the thermal conductivity. Moreover, it is also confirmed by Peng that the thermal conductivity of the magnesium alloy still
Fig. 8. Mg-Ce-Zn alloy phase diagram in Mg -rich corner of 573 K (the green line), 623 K (the blue line) and 673 K (the red line) calculated by Pandat software. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
L.-F. Hu et al. / Journal of Alloys and Compounds 741 (2018) 1222e1228 Table 2 The point calculation results of solid solubility of Ce and Zn in Mg of the studied MgCe-Zn-Zr alloy calculated by Pandat software. wt %
573 K
623 K
673 K
Fraction of Ce in a-Mg (wt %) Fraction of Zn in a-Mg (wt %)
0.00090 0.29641
0.00383 0.33362
0.01302 0.36782
673 K. Moreover, more complete recrystallization played at the extrusion temperature higher than 623 K, which indicates more equiaxed grain would form as well as more grain boundaries. The grain boundary can also be the factor to block the electron and phonons transfer [1] ).
3.4. The mechanical properties of as-extruded alloys The mechanical properties of as-cast, as-homogenized and asextruded are shown in Fig. 9. Compared with the as-casted alloy, the as-extruded one have a higher elongation because of the segregation reduction. The ultimate tensile strain (UTS) and elongation are increased obviously after extrusion. Finer grains play an important role in improving the mechanical properties of asextruded alloys according to Hall-Petch formula. With the increasing extrusion temperature, the UTS of the Mg-Ce-Zn-Zr alloys rose first and then decreased while the elongation was inverse. Complex mechanism based on work hardening, recovery and recrystallization during the process of extrusion should be responsible for the change of UTS refer to [34]. Work hardening works at low extrusion temperature as mentioned before while recovery and recrystallization act at higher extrusion temperature. Work hardening works in Mg-Ce-Zn-Zr alloys when the extrusion temperature was lower than 623 K. Dislocation combined with refined grains lead to higher strength and lower elongation. When the dislocation caused by work hardening disappeared and the grain grows heavily in the extrusion temperature higher than 623 K, the strength decreased whereas the elongation increased.
4. Conclusion The influence of microstructure evolution on mechanical properties and thermal conductivity of Mg-Ce-Zn-Zr alloy extruded at different temperatures is investigated. Based on the analysis results, the conclusions are obtained as follows:
Fig. 9. Tensile testing results of as-casted, as-homogenized and as-extruded samples.
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1. Two phases Mg matrix and (Mg, Zn)12Ce existed in the as-cast alloy. Zr-core appeared and grain boundary interrupted with the homogenization treatment at 713 K for 12 h, and the (Mg, Zn)12Ce transforms to (Mg0.75Ce0.25)O2. 2. With the increase of extrusion temperature, the Zr-containing bar is broadened and the texture of alloy is weakened due to recovery, recrystallization and grain growth, the thermal conductivity of extruded alloys changed from isotropic to anisotropy. The thermal conductivity of Mg-Ce-Zn-Zr initially increased and then decreased when the extrusion temperature increased from 573 K to 673 K because of the disappearance of dislocation and the increased solid solution of Ce. 3. Compared with the as-cast and as-homogenized state, the grain refined with the process of extrusion which contributes to a higher UTS and elongation. The UTS in the longitudinal direction of extruded alloys rose first and then decreased with the increase of extrusion temperature, while the change of elongation is inverse, due to the work hardening, grain refinement, and growth. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51474143, 51674163 and 51074103) and Science and Technology Committee of Shanghai under No. 16ZR1412000. The authors gratefully acknowledge support for experiment materials from Baotou Research Institute of Rare Earths, material analysis and research from Instrumental Analysis and Research Center of Shanghai University, SR-PXRD analysis from Powder diffraction beamline, Australian Synchrotron. References [1] J. Peng, L. Zhong, Y. Wang, et al., Effect of extrusion temperature on the microstructure and thermal conductivity of Mge2.0Zne1.0Mne0.2Ce alloys, Mater. Des. 87 (6) (2015) 914e919. [2] C. Wang, Z. Cui, H. Liu, et al., Electrical and thermal conductivity in Mge5Sn alloy at different aging status, Mater. Des. 84 (2015) 48e52. [3] I. Ostrovsky, Y. Henn, Present State and Future of Magnesium Application in Aerospace Industry, 2007. [4] M. Galeazzi, D.F. Bogorin, K. Prasai, et al., Note: thermal properties of magnesium in the 60-150 mK range, Rev. Sci. Instrum. 81 (7) (2010), 076105. [5] H. Yu, Y.M. Kim, B.S. You, et al., Effects of cerium addition on the microstructure, mechanical properties and hot workability of ZK60 alloy, Mater. Sci. Eng. 559 (598) (2013) 798e807. [6] G. Song, D. StJohn, The effect of zirconium grain refinement on the corrosion behaviour of magnesium-rare earth alloy MEZ, J. Light Met. 2 (1) (2002) 1e16. [7] H. Haferkamp, F. Bach, V. Kaese, et al., Magnesium corrosioneprocesses, protection of anode and cathode, Magn. Alloys Technol. (2004) 226e241. , M. Stane k, P. Luka [8] A. Rudajevova c, Determination of thermal diffusivity and thermal conductivity of Mg-Al alloys, Mater. Sci. Eng. A 341 (1e2) (2003) 152e157. [9] Y.M. Kim, S.W. Choi, S.K. Hong, The behavior of thermal diffusivity change according to the heat treatment in Al-Si binary system, J. Alloys Compd. 687 (2016) 54e58. [10] T. Ying, H. Chi, M. Zheng, et al., Low-temperature electrical resistivity and thermal conductivity of binary magnesium alloys, Acta Mater. 80 (2014) 288e295. [11] J. Yuan, K. Zhang, X. Zhang, et al., Thermal characteristics of MgeZneMn alloys with high specific strength and high thermal conductivity, J. Alloys Compd. 578 (6) (2013) 32e36. [12] L. Zhong, J. Peng, M. Li, et al., Effect of Ce addition on the microstructure, thermal conductivity and mechanical properties of Mge0.5Mn alloys, J. Alloys Compd. 661 (2016) 402e410. [13] J. Peng, L. Zhong, Y. Wang, et al., Effect of Ce addition on thermal conductivity of Mge2Zne1Mn alloy, J. Alloys Compd. 639 (5) (2015) 556e562. [14] T. Walde, H. Riedel, Simulation of earing during deep drawing of magnesium alloy AZ31, Acta Mater. 55 (3) (2007) 867e874. [15] R.K. Mishra, A.K. Gupta, P.R. Rao, et al., Influence of cerium on the texture and ductility of magnesium extrusions, Scripta Mater. 59 (5) (2008) 562e565. [16] R.K. Sabat, R.K. Mishra, A.K. Sachdev, et al., The deciding role of texture on ductility in a Ce containing Mg alloy, Mater. Lett. 153 (2015) 158e161. [17] N. Stanford, D. Atwell, A. Beer, et al., Effect of microalloying with rare-earth elements on the texture of extruded magnesium-based alloys, Scripta Mater. 59 (7) (2008) 772e775.
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